The biogenesis of a respiratory competent mitochondrion results from the joint expression of two distinct genetic systems, mitochondrial and cytoplasmic. As yet the means by which mitochondrial protein synthesis is coordinated with cytoplasmic protein synthesis remains unclear. In this study, yeast cytochrome c oxidase, a definable entity of the inner mitochondrial membrane which is composed of polypeptide subunits translated on both protein synthetic systems, is proposed as a prototype system with which to study this coordination. Subunit-specific antisera and purified subunits will be used to study the time of synthesis of both sets of subunits during the cell cycle, the role of post translational processing in subunit assembly and the stimulatory effects of cytoplasmic translation products on the in vitro translation of mitochondrially-made subunits. These studies will be complemented by in vitro reconstitution experiments designed to assess the role of hydrophobic mitochondrially-made subunits in enzyme assembly and in the binding of hydrophilic cytoplasmically-made subunits to the membrane.